The manufacture of rolled metal parts requires primarily that ingots, bars or plates of steel or other metals, are roll formed through a succession of high pressure rollers, each reducing the base stock to a successively smaller cross-section until the desired dimension/shape is attained. These extreme pressure processes traditionally use large volumes of sulfonated/sulfated and/or chlorinated mineral oil or other mineral oil derivatives with extreme pressure additives to lubricate and cool the metal during the rolling process.
In the manufacturing of cast metal parts, the parts are cast from molten metal in foundry molds then machined to the desired shapes and dimensional tolerances. Lubricants are used to cool and lubricate the machine tools and the metal surfaces during the machining.
In the manufacture of powdered metal (sintered) parts it is frequently necessary to restrike or reform the parts after sintering in order to achieve specific dimensional tolerances desired or to compress the part to a desired higher density. This process is identified by various terms including "sizing", "re-striking", "coining", "burnishing" or "qualifying". Lubricants are used on the surface of the powdered metal parts in processing the parts before sintering and in sizing the parts after sintering. Lubricants are further used in machining and burnishing the parts.
Sizing requires the part to be placed in a steel or carbide die and squeezed under extremely high pressure to produce the specified dimensional and/or density requirements. This process requires extreme pressure boundary layer lubricants at the outset of the sizing stroke, followed by anti-weld lubricants as the heat and pressures tend to act to gall the workpiece and weld the exfoliative to the surfaces.
Typical lubricants are mineral oils and synthetic oils such as polybutenes, .alpha.-olefins and polyethyleneglycols. These oils do not have strong polar groups and they are relatively low in lubricating ability. Accordingly, they cannot be used as a lubricant by themselves. Therefore, oiliness improvers, for example fats, saturated and unsaturated fatty acids, fatty acid esters, phosphates and alcohols are used to improve the lubrication properties of these oils. However, under extreme pressure applications, the oiliness improver is not effective and an extreme pressure additive such as sulfur, chlorine, phosphorus or lead is necessary. If chlorine is added and water is present during processing HCl is liberated causing serious corrosion problems. Furthermore, these oils are not suitable for making parts of higher densities with higher sizing pressures, because such oily lubricants tend to cause burning.
In the sizing process, a portion of the lubricant enters the pores of the powdered metal part, and other portions of the lubricant may be redeposited on the surface during ejection and handling. If the lubricant is an oil, excessive amounts entering the pores of the parts may prevent the part from compressing (a phenomenon known as hydraulicing) and may damage the dies.
U.S. Pat. No. 4,086,087 to Morris discusses the problem of oil in powdered metal parts and teaches that the pores be filled in a pretreatment step with an immiscible liquid, such as water, prior to contacting the metal part with the lubricant. The immiscible liquid is intended to prevent the oil lubricant from entering the pores, but excess water in the pores can also cause hydraulicing. It would be advantageous to have a lubricant which avoids this problem of oil based lubricants.
Because of the difficulty with oil lubricants, a common practice is to dry-coat the parts before sizing. The dry coating may be zinc-stearate, calcium carbonate or a similar dry lubricant which is suspended in a highly volatile carrier such as alcohol or trichloroethane. These lubricants are adequate in the dry form for medium pressure lubrication. In extreme pressure dry sizing, historically the lubricant has been a siloxane suspended in 1,1,1,-trichloroethane or a similar highly volatile solvent. The latter category of dry lubricant is generally considered the most successful to date for extreme pressure applications. Such dry lubricants are often supplemented with the addition of graphite powder. A significant problem of dry lubricants is that they have no cooling capacity.
In all cases the lubricant must also be compatible with subsequent operations. In the case of mineral oil based lubricants, and some dry lubricants, they generally must be removed before the part can be subjected to subsequent processing. This removal process may require burning off the lubricant in an oven at about 800.degree. to 1000.degree. F., or vapor degreasing the parts with chlorinated solvents, neither of which is desirable. The subsequent processes which necessitate the removal of the lubricants include, but are not limited to, heat treating, steam oxidizing, and resin impregnation.
Many powdered metal parts are subject to secondary operations which include but are not limited to drilling, tapping, honing, milling, broaching, lapping, and turning. Each of these operations may require a unique coolant or lubricant with different performance parameters capable of cooling the part and tools as well as providing corrosion control for the parts and equipment. For example, honing oils often require a high sulfur content; machining coolants might be semi-synthetic or synthetic lubricants; tapping lubricants require chlorinated oils. Each of these has its characteristic advantages and disadvantages.
A powdered metal parts manufacturer may have as many as a dozen different specialty purpose lubricants and rust inhibitors, each requiring special operator training and storage and disposal considerations. In view of the above, it would be advantageous if one lubricant could replace all of the oils, dry lubricants and coolants for these operations.
In addition to application problems with extreme pressure lubricants there are serious environmental problems with the use and disposal of solvents and oils. The products based on mineral oil cause oil smoke and oil mist in the work-room and air quality problems in and around the machines.
Although a water based lubricant generally dissipates from heat before total compression, an excess of fluid can cause hydraulicing. Moreover, if the lubricant is water based, it must have adequate corrosion control additives in order to survive the heat and pressure and still provide sufficient corrosion control until the parts are processed to the next operation.
Accordingly, there is a need to find an environmentally acceptable high performance metal working lubricant. The lubricant must demonstrate good lubricating and cooling ability at high surface pressures and/or good cutting and conversion velocities to give products the desired conformation, tolerance and surface finish, as well as decreased wear of the tools. Additionally, there is a need to find a metal working lubricant which can be used in a variety of the functions and processes of metal working, metal forming and metal finishing.
Accordingly, the present invention provides lubricant compositions that unexpectedly meet these stringent requirements. The compositions of this invention may also be used in many other applications such as conventional metal working, textile processing, paper processing and hydraulic systems.